Food and milk from the offspring of cloned animals may have entered the U.S. food supply, the U.S. government said on Tuesday, but it would be impossible to know because there is no difference between cloned and conventional products.

The U.S. Food and Drug Administration said in January meat and milk from cloned cattle, swine and goats and their offspring were as safe as products from traditional animals. Before then, farmers and ranchers had followed a voluntary moratorium on the sale of clones and their offspring.

While the FDA evaluated the safety of food from clones and their offspring, the U.S. Agriculture Department was in charge of managing the transition of these animals into the food supply.

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“It is theoretically possible” offspring from clones are in the food supply, said Siobhan DeLancey, an FDA spokeswoman.

Cloning animals involves taking the nuclei of cells from adults and fusing them into egg cells that are implanted into a surrogate mother. There are an estimated 600 cloned animals in the United States.